Optical isolators are optical components that transmit light in one direction but block it in the backward-propagating direction. They are used when the reversely-propagating light needs to be avoided. For example, lasers become instable when reflected light comes back to them.
Conventional optical isolators consist of a Faraday rotator and two polarizers. The linear-polarization angle needs to be rotated by 45 degrees in a Faraday rotator, which is typically a few millimeters long. Thus, this type of optical isolator becomes relatively large. It is also a challenge to build Faraday rotators and polarizers in a guided-optics format, i.e. it is hard to integrate them on chip. In order to construct guided-wave optical isolators, an asymmetric Mach-Zehnder waveguide interferometer is commonly used. This optical isolator does not require polarizers, but requires high-precision interference for blocking backward propagating light waves; one branch of the interferometer is a nonreciprocal phase shifter, and the other is a reciprocal phase shifter. However, this optical isolator still requires two long waveguides.